The present invention relates to an improved method for finishing the surfaces of metallic objects. More particularly, the present invention provides an improved dry processing method for tumbling metallic objects to provide a smoothly polished finished surface while providing protection to recessed areas of the object that are not intended to be polished.
Presently, to provide a polished finished on a relatively small object, two processes are available, a dry process and a wet process. The xe2x80x9cwetxe2x80x9d process involves placing abrasive polishing media and a mixture of turpentine and naphtha into a tumbling drum and adding the objects to be polished. The drawback to this wet process is the quantity of waste tumbling media and volatile chemicals that remain after the process is completed. These hazardous materials must be properly disposed of after use. In addition, the entire surface of the object must receive the same level of finish.
In the xe2x80x9cdryxe2x80x9d tumbling process, hard pellets, such as hard wood pegs, are first prepared by coating them with a variety of oils, waxes, abrasive agents and adhesives. The pellets are then loaded into the tumbling drum and then the objects to be finished are added. The tumbling action generates friction and heat that allows some of the waxes and oils to soften and act as a lubricant for the tumbling of the objects. The friction of the objects and the pellets colliding in the tumbling action in effect polishes the surface of the finished objects while generally being gentle enough not to damage the objects. The advantage to this process is the elimination of the wet material that must be disposed of as hazardous waste. However, differential surface finishes still cannot be provided.
The drawback with the present state of the art is that objects finished in both the xe2x80x9cwetxe2x80x9d and xe2x80x9cdryxe2x80x9d manner must have a uniform finish across the entire surface due to the impact and rubbing friction involved. Objects that have differentiated surfaces such as recesses, or objects that are desired to have a different finish on part of their surface must be entirely finished by hand. For example, a ring or bracelet that has a recessed design with a frosted finish on the recessed surfaces and a polished finish on the raised surfaces cannot be tumbled, but must first be sandblasted to texture the recessed surfaces and then hand polished on a buffing wheel to shine the raised surfaces. This process requires a great deal more labor and cost to provide the desired finished objects.
Another drawback to the tumbling process is that over time, the process becomes inconsistent and requires extended tumbling times to achieve a polished finish. This is a result of the build up of waxes and oils on the tumbling media as the tumbling media is reused.
In view of the foregoing, there is a demand for an automated polishing method that can handle a large number of objects to be polished in bulk. There is a demand for a dry bulk process by which objects that have differentiated surfaces can be polished while allowing an earlier applied finish to remain on selected areas of the surface. In addition, there is a demand for a dry bulk polishing system that provides a consistent finish while providing for reuse of the tumbling media.
In this regard, the instant invention provides an improved method of polishing metallic objects that allows differential finishes to be provided on the surface of the object while providing a bulk finishing operation. In particular, this method has application in the jewelry finishing field as often different finishes such as sandblasting, brushing or polishing are used in combination to achieve the desired appearance on the finished piece. By way of example, the present invention will be illustrated as applied to fine gold or sterling silver jewelry and in particular a bracelet, although the application and scope of the present invention is not limited by this illustration.
The present invention is employed to provide a differential surface finish on cast metallic items such as jewelry. Once the item is removed from the casting mold the first finish is applied on the recessed surfaces of the object. To overcome the drawback of the prior art and to protect the first finish a protective barrier is applied over the entire piece. The protective barrier consists of a water-soluble stop-off material. The piece is then buffed by hand, removing the stop-off material from the raised surfaces of the piece but leaving a protective barrier over the finish applied in the recessed areas.
Once several pieces are prepared as described above, they are loaded into a tumbling drum with a tumbling media and a high VOC dry tumbling compound. The parts are tumbled and the friction generated with the tumbling media polishes the exposed surfaces of the objects, while the recessed surfaces remain protected beneath the stop-off material. The tumbled pieces are then removed from the drum for final cleaning and finishing, and the tumbling media within the drum is cleaned for reuse in subsequent tumbling operations in accordance with the present invention. In particular, the media is cleaned by rotating the drum at a high rotational speed to cause the more volatile polishing compounds to xe2x80x9cflash offxe2x80x9d, taking with them the heavier polishing materials and leaving the tumbling media clean for the next batch of objects.
The tumbled jewelry pieces are cleaned using an ultrasonic water spray to break down and remove the water-soluble stop-off material, exposing the textured finish on the recessed surfaces. The pieces are then lightly buffed by hand and prepared for shipment.
Accordingly, among the objects of the instant invention are: the provision of an efficient bulk process for imparting a differential finish to a metallic object; the provision of a dry polishing process capable of providing a differential finish; the provision of a dry polishing process that also provides for reuse and conservation of the tumbling media while producing consistent results.